1. Field of the Invention
The present invention relates to an antenna device, and more particularly to a horn antenna device with a step-shaped signal feed-in apparatus for focusing electromagnetic waves.
2. Description of Related Art
In order to measure an object level, such as water level in a reservoir or mineral stockpiles of a quarry, a radar level meter is developed to measure the object level.
The radar level meter can be installed at a position far from an object. The radar level meter mainly comprises a circuit board and an antenna. The circuit board generates electromagnetic waves of FMCW (frequency modulated continuous waves) and radiates the electromagnetic waves toward the object through the antenna. The electromagnetic waves are then reflected from a surface of the object when the electromagnetic waves reach the object. Afterward, the circuit board can receive a reflection of the electromagnetic waves from the antenna.
The circuit board has a controller. The controller calculates a frequency difference and a time difference between the reflection and the electromagnetic waves emitted from the antenna. When the controller obtains the frequency difference and the time difference, the controller calculates a distance between the radar level meter and the surface of the object or further calculates an object level according to the distance.
With reference to FIG. 9, a conventional level measuring device is disclosed. The measuring device comprises a radar level meter 80 and a waveguide apparatus 90. The waveguide apparatus 90 has a body 91, a medium converter 92 and a feed-in connector 93. The body 91 has an opening 911, a hole 912 and a space 913. The space 913 communicates with the opening 911 and the hole 912. The medium converter 92 is conical in shape and has a cone point 921 and a bottom end 922 distal to the cone point 921. The bottom end 922 of the medium converter 92 is mounted in the opening 911 of the body 91. The feed-in connector 93 can be an SMA connector or an SMP connector and is inserted in the hole 912. The feed-in connector 93 has a probe 931 extending into the space 913 of the body 91.
The radar level meter 80 is connected to an end of the waveguide apparatus 90 distal to the probe 931. The radar level meter 80 feeds high frequency electromagnetic waves into the space 913 of the body 91 through the feed-in connector 93. The medium convertor 92 thus radiates the electromagnetic waves outward. The medium converter 92 is used for matching impedance. The medium converter 92 converts a spherical wave to a plane wave beneficial for transmitting and receiving wireless signals.
However, the electromagnetic waves generated from the feed-in connector 93 are fed in the medium converter 92 via the space 913. The electromagnetic waves are not directly fed in the medium converter 92. As such, the resonating modes, such as TE mode and TM mode, of the electromagnetic waves are difficult to be determined. Correspondingly, it is complicated and highly time-consuming to design the structures of the waveguide apparatus 90. Assembling problems, such as the feed-in connector 93 not exactly mounted in the hole 912 and the medium convertor 92 not securely mounted to the body 91, result in inadequate stability.
With reference of FIG. 10, a reflection coefficient (S11) chart of the electromagnetic waves generated from the waveguide apparatus 90 is disclosed. The return loss at 9.0 (GHz) is −12.5 (dB). The return loss at 9.5 (GHz) is −15.846 (dB). The return loss at 10 (GHz) is −13.285 (dB). When the return loss is lower than −10 (dB), a band width is only 1.75 (GHz), wherein 10.5 (GHz)−8.75 (GHz)=1.75 (GHz). A radiation pattern of the medium convertor 92 is easily affected and thus lowers the signal quality.